Only a handful of the planets in our solar system sport planetary rings and various theories encompass their formation. Nevertheless, planetary scientists admit that we have very little understanding of the mechanisms that form them.
So how do they actually form? It's challenging to say. Developing a unified planetary ring formation theory isn't easy because the composition of planetary rings varies from one planet to the next.
Jupiter and Uranus, for example, each sport planetary rings comprised of dust and rock, while Saturn's, on the other hand, appears to be made of small fragments of ice. Given just how different these compositions are, it's safe to say these planets didn't develop their rings in the same manner.
How these rings formed may continue to fuel debate in the scientific community for years to come, but without observing the formation in the process, there's no concrete answer. This is one of the reasons why astronomers search the cosmos far and wide for exoplanets - we just might get lucky one day.